Heretofore, control techniques for clothes dryers have utilized time drying periods during which moisture is removed from the clothes to be dried by passing heated air over the clothes for a fixed period of time. This resulted in a drying period which may have been longer or shorter than was needed to properly dry the clothes.
Another clothes drying technique which has been used in that of controlling the drying period according to a direct or indirect method of determining the moisture content of the clothes. In the direct method, a resistance sensor is utilized to monitor the resistance of the clothes as they are dried. As moisture is removed from the clothes, the sensed resistance increases, and the heating cycle of the drying period is terminated upon detection of a predetermined resistance. The principle employed in such a method is that the sensed resistance will indicate the percent dryness of the clothes. In the known indirect method of controlling drying of clothes, exhaust air temperature is sensed to determine percent dryness. In the indirect method it is assumed that air temperature remains relatively constant while heat input goes into changing the phase of water, i.e., evaporating it from the clothes. The indirect method operates on the principle that once the moisture is driven off, heat input goes into raising the temperature of the drying air. In one known type of indirect drying system the temperature increase is detected by a bimetal thermostat which operates at a fixed predetermined temperature.
In each of the types of clothes drying techniques just described, a cooling cycle comprising a predetermined timed portion of the drying period has been utilized to lower the temperature of the clothes. Such a timed cooling cycle has the same disadvantages of a timed heating cycle in that the clothes may not be sufficiently cooled (thereby resulting in an uncomfortable or unsafe condition when clothes are to be removed from the dryer); or, if the timed cooling cycle is in excess of that required to cool the clothes to the proper temperature, unwanted wrinkles may be set in the fabric (especially with fabrics made from certain synthetic fibers).